Sulphur is regarded as an essential nutrient for plant growth therefore low crop yields and poor quality crops may often be attributed to deficiencies in sulphur levels in the soil.
Separately, some soils, such as alkaline soils, sodic soils, or sodic alkaline soils, have characteristics that make them unsuited or sub optimal for agriculture. Correcting the pH of such soils and/or restoring the ionic imbalance in the soil, and/or correcting both the pH and the ionic imbalance of the soil can restore the suitability of soils for agricultural use.
Accordingly, the use of sulphur-based fertilizers and soil improvers has been required in order to restore the soils for agricultural use.
Generally the nutrient value of sulphur and its acidifying characteristics is dependent upon it being in the form of sulphates, but a disadvantage of sulphates if applied directly is that they are mobile in the soil and easily leach out of the root zone, effectively making the sulphate nutrient unavailable to the plants.
Elemental sulphur is known for use as a fertilizer or soil improver and is advantageous in that it is able to remain in the soil over extended periods gradually being degraded by soil bacteria into the plant-usable sulphate form. In this way, elemental sulphur can be regarded as a slow (timed) release form of plant nutrient that is less prone to leaching from the root zone. Elemental sulphur also offers further benefits in that it may also act as a fungicide against certain microorganisms, as a pesticide against certain soil and plant pests, and assists in the decomposition of plant residues. A particular benefit is that it may also improve phosphorus and nitrogen nutrient utilization in alkaline and calcareous soils.
Application of elemental sulphur is not without problems, however, as to be readily degradable in the soil it should be utilized in finely divided form. As will be appreciated, finely divided sulphur presents an explosive hazard and may release toxic gaseous impurities.
Elemental sulphur dispersions can be produced by mixing dry sulphur powder, such as milled sulphur or flowers of sulphur, in a dispersion medium, or by dispersing pre-aggregated sulphur particles in a dispersion medium. In each case, the preparation, transport and handling of free flowing or agglomerated sulphur powder presents a safety risk.
It is also known that finely divided elemental sulphur particles can be released from formed elemental sulphur products that contain swelling agents. The agronomic efficiency of such products relies upon the interaction between moisture and the swelling agent; the moisture causing the swelling agent to grow in size resulting in the product breaking down into sulphur particles of various sizes. Such a technique involves the particle size of sulphur to be typically one order of magnitude larger than that of sulphur prepared by milling techniques and hence the sulphur is less agronomically efficient. Moreover, the product is highly hygroscopic and hence requires special care for handling, transportation, storage and application in order to avoid breaking up of the material and the formation of explosible sulphur dust.
Wet milling techniques in the presence of additives are also known for preparation of sulphur suspensions but these products too have drawbacks in that when stored in closed containers they are prone to gradual release of amounts of chemicals dissolved or chemically bound to sulphur such as hydrogen sulphide and sulphur dioxide. Slow release of such gases can occur even when degassed sulphur, generally known to contain <10 ppm hydrogen sulphide, is used as the sulphur source. The concentration of these noxious gases increases over time when stored thus placing users at risk. While it is possible to achieve adequate fixation of suspensions containing hydrogen sulphide and sulphur dioxide, in doing so handling of these gases is required, hence there is still a risk of exposing personnel to the noxious gases. Moreover, it requires the involvement of a separate processing step during handling, transport, or application of the suspension, none of which is desirable.
Accordingly, there remains a need for an improved process for preparing sulphur-based compositions for application as fertilizers and/or soil improvers. In particular, the present invention seeks to provide a safe and efficient method of producing finely divided sulphur compositions which are convenient and non-hazardous to handle, transport and apply.